The present invention concerns the field of telemetric systems, in particular systems for measuring distances. More particularly, the present invention relates to a digital correlation device, used in such telemetric systems, which allow the occurrence of a coded reference sequence to be detected in a sampled reception signal, such occurrence allowing, in particular, the distance from a target to be determined. The present invention also concerns a correlation method.
Telemetric distance measuring systems based on signal compression usually include means for transmitting a modulated signal by means of a coded reference sequence, a first antenna adapted to direct the power from the signal towards a target, a second antenna adapted to receive the power reflected by the target, and a correlation device determining the amount of delay which occurs on the reception signal and thus the distance of the target. More specifically, this delay is determined by correlating the coded reference sequence with the reception signal for a set of delays thereof. The presence of a correlation peak among the correlation values calculated allows the delay corresponding to the distance of the target to be identified.
The signal modulation technique typically used in such telemetric systems is called spread spectrum modulation. This modulation technique consists in spreading the bandwidth of the signal in order, in particular, to minimise the sensitivity of the latter to white noise or jamming, and finds application in various fields such as telemetry, radar systems or telecommunications. In particular, this modulation technique is applied to telemetric systems, such as the distance measuring systems to which the invention relates.
Various spread spectrum modulation techniques are known to those skilled in the art. In particular, a technique consisting in modulating the signal transmitted towards the target by means of a coded pseudo-random sequence is known by the name of direct sequence spread spectrum modulation or pseudo-noise spread spectrum modulation. The present invention essentially concerns this type of modulation technique.
The use of pseudo-random sequences in telemetric applications has mainly been applied to long distance telemetry. Examples of these applications are for example the GPS system or atmospheric analysis systems. This is explained mainly by the fact that the resolution of such systems, in terms of distance, is directly linked to the sampling frequency of the reception signal. For short distance applications, higher resolutions are necessary and, consequently, higher sampling frequencies requiring a very rapid hardware system implementation.
The digital correlators currently available on the market have limitations both in terms of correlation speed and code length. In particular, in order to implement such a telemetric distance measuring system, the correlators must have a complementary parallel peak detection function able to detect the correlation values for different delays of the reception signal. This function is generally achieved by means of specific circuits which, however, cannot operate at the speed required for short distance applications.
It is also possible to use a serial peak detector, also called a sliding correlator, but the acquisition time of this system, i.e. the time necessary to find the correlation peak, is much too long and thus makes the use of such a peak detector unsuitable for distance measuring applications.
One object of the present invention is thus to propose a digital correlation device able to operate at the high correlation speeds required for short distance applications.
Another object of the present invention is to propose a digital device whose architecture is substantially independent of the code length of the reference sequence used for correlation.
The invention therefore concerns a digital correlation device allowing detection of the occurrence of a coded reference sequence including N code elements or chips in a sampled reception signal, this device including the following features: (1) first and second registers for respectively loading successive samples of the reception signal and chips of the coded reference sequence, and (2) calculating means delays of the reception signal, wherein (3) the reception signal is sampled a frequency equal to d times the chip rate of the reference sequence; (4) the device is arranged to generate dxc3x97Nd correlation values corresponding to dxc3x97Nd successive delays of the reception signal, Nd being a lower number than the number N of chips of the reference sequence, each correlation value, for a given delay of the reception signal, being obtained at the end of a determined number of partial correlation operations during which the calculating means generate a partial correlation value covering code portions including Nd successive chips of the reference sequence; and (5) the device further includes accumulation means, coupled to the calculating means, and including dxc3x97Nd memory sections each allowing the partial correlation value generated for a given delay of the reception signal to be accumulated during each partial correlation operation.
Variants of this digital correlation device are the subject of the-dependent claims.
The present invention also concerns a novel correlation method allowing detection of the occurrence of a coded reference signal including N code elements or chips in a sampled reception signal, this method generating correlation values for a set of successive delays of the reception signal, wherein: the reception signal is sampled at a frequency equal to d times the chip rate of the reference sequence; and dxc3x97Nd correlation values are generated for dxc3x97Nd successive delays of the reception signal, Nd being a lower number than the number N of chips of the reference sequence; this method including the steps of: (a) selecting a code portion covering the Nd first successive chips of the reference sequence; (b) selecting a set of samples of the reception signal corresponding to the first delay; (c) calculating a partial correlation value between the selected code portion and the selected samples of the reception signal; (d) accumulating the calculated partial correlation value; (e) selecting a set of samples of the reception signal corresponding to the next delay; (f) repeating steps (c) to (e) until a partial correlation value has been calculated and accumulated for the dxc3x97Nd delays of the reception signal; (g) selecting a code portion covering Nd successive chips of the reference sequence directly following the previously selected code potion; and (h) repeating steps (b) to (g) until the selected code portion covers the Nd last successive chips of the reference sequence.
One advantage of the correlation device according to the present invention lies in the fact that its architecture, which is simple to realise, allows it to be used at higher sampling frequencies than previously possible with the correlation devices of the prior art, and thus its use is perfectly suited to short distance telemetric systems.
This simple architecture further allows the cost of the correlation device to be substantially reduced and, thereby, the cost of the telemetric system in which it is used.